The present invention relates to a melting furnace for treating industrial wastes, particularly high-level radioactive wastes, and a heating method of the melting furnace.
In order to treat industrial wastes, particularly high-level radioactive wastes, there has been utilized heretofore a method wherein the wastes are put into a molten glass and resulting mixture is cooled to form a vitrified product immobilizing the wastes therein.
The conventional glass melting furnaces for this purpose can be classified broadly into a metallic melting furnace and a refractory melting furnace according to the kind of wall materials that are in contact with molten glass.
For the metallic melting furnace, there have been generally employed an external heating method wherein the glass inside the furnace is heated from the outside of the furnace by intermediate frequency induction heating or by resistance heating, and a microwave heating method wherein the glass inside the furnace is heated directly by applying microwaves. On the other hand, for the refractory melting furnace, an electric power supply heating method wherein the glass inside the furnace is heated by supplying electric power through electrodes has been employed.
However, the conventional metallic melting furnaces generally involve the problems that the metallic wall materials in contact with molten glass are corroded and, since this corrosion is promoted by heating, the service life of the furnace is shortened. In contrast, the refractory melting furnace can have its service life extended by selecting wall materials having high corrosion resistance such as chromia-alumina type refractory bricks or the like, but is not free from the drawback that the secondary wastes such as the wall materials after the end of the service life are generated in large quantities. It has been reported that 80 g of secondary wastes per 1 Kg of glass were generated in a melting furnace of this kind (C. G. Sombret, "Melters and Furnace Equipment Used for Radioactive Waste Conditioning" in Proceedings of the 1987 International Waste Management Conference, Hong Kong, Nov. 29-Dec. 5, 1987, p259).
In accordance with the external heating method employed for the metallic melting furnace, on the other hand, the necessary energy is supplied to the glass inside the furnace by heat transfer through the wall surface that is in contact with the glass to be melted. Therefore, it is difficult to increase the contact surface area in proportion to the increase of the processing capacity after a certain processing capacity has been achieved and, thus, there is an inevitable limit to the processing capacity per unit volume of the furnace. For example, it has been reported in the above-described reference that the maximum processing capacity per unit vitrification plant for treating high level radioactive wastes in France was 35 kg glass/hr. In order to treat the radioactive wastes on an industrial scale, therefore, a plurality of melting furnaces must be installed and if they are installed, there occurs another problem that the operation becomes complicated.
The microwave heating method involves the problem that if the melting furnace is large in scale, only the surface portion of the glass charged into the furnace is heated but the inner portions thereof cannot be melted sufficiently. Therefore, this method is not suitable, either, for the industrial treating method of the radioactive wastes.
There is also a problem in the power supply heating method which has been employed for the refractory melting furnace. Namely, in the processing of high-level radioactive wastes generated during the reprocessing of spent nuclear fuels of a nuclear power plant, for example, a radioactive waste containing therein platinum group metals is produced. When such a radioactive waste is heated by the power supply heating in the refractory melting furnace, electrically conductive materials which are difficult to be dissolved in the molten glass are produced and deposited at the bottom of the furnace. Since the current flows concentratedly through the conductive materials deposited at the furnace bottom, the current efficiency drops and the processing performance of the furnace is caused to deteriorate.